I got my first Tetris 99 just now, and I only picked it up a couple hours ago.
I seem to do best when I start in KO mode, then just stay in attackers the rest of the time to focus on surviving. I get consistent high placement that way.

As long as I'm not making mistakes from rushing, I'm doing fine.
On occasion, I've had bad inputs because it didn't buffer my rotation, but then go straight for a hard drop.
I also keep mistaking what my next piece is on occasion, it's weird. I don't get to get that in Puyo Puyo Tetris.

I can't imagine Tetris is a particularly server intensive game, it doesn't require a great deal of precision of synchronisity between players.
Although it is displaying the playing field of 99 players simultaneously, the information is limited to a 10x20 grid (200 spaces) where each block has 8 possible states, which can be stored in a 3-bit value (.e.g 000 empty, 001 red, 010 green, 011 yellow, 100 blue, 101 orange, 110 purple, 111 cyan)
With 98 other playing fields to display, this amounts to 19600 blocks to track, I guess amounting to 58,800 bits of data per tick, or 7350 bytes, or less than 8kb.
And that's without accounting for the frequency of updates or ticks, there will be ticks with no update so the package size would be smaller than 8kb per tick, and it will only decrease over time as players get knocked out.

This isn't accounting for other variables like their target behaviour or the piece they are moving.

I was browsing eShop couple of hours ago and noticed that my gold points had arrived. Got an email about it just now too. I hope they have more events like this. It was fun to compete, especially since there was a good chance of winning, at least in Europe.

I programmed a simple tool that copies all Switch screenshots from an SD card and places them to folders according to the game:Link

Yup, it seems like they're distributing their rewards to My Nintendo entrants now, as well as a kudos for those who did not win the 999 Gold Points.
You can see it in the alerts on the My Nintendo page.

@Daloblast
I've only played 54 games and won 13 of them.
Skill level is very much a factor. What targeting you use it somewhat important, but I've had a win where I just left it on random the entire time.